Wheel assembly for vehicle having drive plates in which forces exceeding a level are not transferred to the axle

ABSTRACT

A wheel assembly for a vehicle such as a tractor is disclosed. The assembly comprises a wheel rim ( 10 ) for supporting a tire, a nave plate ( 11 ) mounted to the rim, and a drive plate ( 21 ) mounted to an axle flange ( 26 ). A torque ring ( 18 ) has first key ways ( 16, 17 ) for cooperating with first keys ( 14, 15 ) on nave plate ( 11 ), and second key ways ( 19, 20 ) for cooperating with second keys ( 22, 23 ) on drive plate ( 21 ). In this way, torque and radial and axial forces are transferred between the drive plate ( 21 ) and nave plate ( 11 ), but radial or axial forces, tending to remove the assembly from the axle, and exceeding respective predetermined levels, are prevented from being transferred from the nave plate ( 11 ) to the axle.

The present invention relates to a wheel assembly for a vehicle, andrelates particularly, but not exclusively, to a wheel assembly forminimising injury to occupants of a vehicle as a result of an explosionunder the wheel assembly. The invention also relates to a vehicleincorporating at least one such wheel assembly.

A substantial risk to vehicles and their occupants is posed by anti-tankmines buried slightly below the surface of the ground as a result ofongoing or previous warfare. Although military vehicles enteringminefields are generally armoured and therefore protected to a certainextent from the effect of explosions, the occupants of civilian vehiclesencountering such mines are at substantial risk of injury or death as aresult of such mines.

Most anti-tank mines are detonated by means of a substantial loadapplied to the upper surface of the mine, as a result of which a minesusually explodes under one of the wheels of a vehicle passing over themine. A major part of the energy of the explosion is transmitted intothe vehicle through the wheel and its connection to the body of thevehicle, with some energy being transferred to the vehicle as a directeffect of the blast on the body of the vehicle. This latter effect canbe a relatively minor effect, depending upon the shape and weight of thevehicle, and the distance between the body of the vehicle and the bottomof the wheel.

However, in a conventional wheel and axle assembly, the wheel is veryfirmly attached to the vehicle by a very strong axle and suspension. Itis necessary for the axle and suspension to be strong because the mainforces between the vehicle and the ground are transmitted through thisassembly. However, if an anti-tank mine explodes under one of thewheels, the same axle and suspension transmits a significant amount ofenergy from the blast to the body of the vehicle and can causesignificant injury or event death to its occupants.

Vehicles intended for use where the risk of exploding mines is high mayhave the whole axle and spring and shock absorber assembly fixed to thevehicle chassis so that it can be blown off without damage to thevehicle fixing points. This enables the whole assembly to be replacedeasily, but is generally only applicable to special purpose militaryvehicles, and not applicable to civilian vehicles, such as agriculturalvehicles. This arrangement also suffers from the drawback that thedamage is still extensive and the time taken to repair it isconsiderable.

Preferred embodiments of the present invention seek to overcome theabove disadvantages of the prior art.

According to an aspect of the present invention, there is provided awheel assembly for a vehicle, the assembly comprising:—

-   a wheel rim;-   a first support plate mounted to said rim;-   a second support plate adapted to be mounted to an axle;-   first coupling means, connected between said first and second    support plates, for transferring torque between said first and    second support plates; and-   second coupling means, connected between said first and second    support plates, for transferring radial and axial forces from said    first support plate to said second support plate;    wherein said first and second coupling means are incapable of    transferring, from said first support plate to said second support    plate, radial or axial forces, tending to remove the assembly from    the axle, and exceeding respective predetermined levels.

This provides the advantage that axial and radial forces exceedingrespective predetermined levels, which are the predominant components ofthe forces generated by a mine exploding under a wheel of a vehiclecarrying the assembly, are prevented from being transferred to the axle,and therefore to the vehicle body. This in turn minimises the extent ofinjury to the occupants of the vehicle and the extent of damage to thevehicle.

Said first coupling means may comprise torque transfer means adapted totransfer torque from said second to said first support plate.

Said torque transfer means may include at least one body member havingfirst engaging means for enabling the or each said body member to slidein a first direction relative to said first support plate, and secondengaging means for enabling the or each said body member to slide in asecond direction, not parallel to said first direction, relative to saidsecond support plate.

Said torque transfer means may include at least one body member and aplurality of first linkages pivotably connected between at least onesaid body member and said first support plate, and a plurality of secondlinkages pivotably connected between at least one said body member andsaid second support plate.

The second coupling means may further include at least one firstcoupling member connected between said first and second support platesand adapted to fracture as a result of application thereto of radial oraxial forces exceeding said respective predetermined levels.

The first and/or second coupling means may include a plurality of secondcoupling members adapted to fracture as a result of fracture of at leastone said first coupling member.

Said second support plate may be adapted to be mounted to said axle viaa gearbox.

Said second support plate may define a region of decreasing crosssection in a direction transverse to said axle.

This provides the advantage of reducing the extent to which the assemblybecomes trapped on the axle as a result of an explosion under the wheelassembly.

The assembly may further comprise removable securing means for securingsaid assembly to the axle.

This provides the advantage of enabling the wheel assembly to be moresecurely attached to the axle when the risk of explosion under the wheelassembly is less, for example when the vehicle is travelling on publicroads.

According to another aspect of the present invention, there is provideda vehicle comprising:

-   a chassis;-   a cab for accommodating at least one occupant;-   at least one wheel assembly as defined above; and-   a respective axle provided on the chassis for supporting the or each    said wheel assembly.

Preferred embodiments of the present invention will now be described, byway of example only and not in any limitative sense, with reference tothe accompanying drawings, in which:—

FIG. 1 is a schematic representation of a prior art agriculturaltractor;

FIG. 2 is an exploded perspective view of a wheel assembly of a firstembodiment of the present invention;

FIG. 3 is a cross sectional elevation view of the wheel assembly of FIG.2 carrying a tire;

FIG. 4 is a cross sectional view of part of a wheel assembly of a secondembodiment of the present invention;

FIG. 5 is a cross sectional view of part of the wheel assembly of athird embodiment of the present invention; and

FIG. 6 is an exploded perspective view, corresponding to FIG. 2, of awheel assembly of a fourth embodiment of the present invention.

FIG. 1 shows a prior art agricultural tractor, which is particularlysuitable for operation in a mine field because the chassis and driver'scab are narrow and well separated from the wheels, which are set far outfrom the center of the vehicle on strong axles that offer littleresistance to blasts. The tractor has front and rear wheels, each rearwheel having a tire (1) of rolling radius r, mounted on a rim (2) whichis connected to an axle flange (3), by means of a nave plate (4),through a number of bolts (5) which are arranged in a circle of radiusx. The front axle of the tractor can pivot about a horizontal axis (6),each front wheel being steerable through a mechanism (7) and having agearbox (8) protruding out beyond the corresponding nave plate.

The external forces acting on the rear wheels come from the ground, andmay be conveniently divided into a vertical component V, a horizontalthrust H and a sideways force S. These forces are reacted from thetractor through the corresponding axle flange (3) by means of the bolts(5).

The tire (1) is designed to achieve a very good grip on the ground, andthe coefficient of friction is therefore high, and may be as high as 1.For the purposes of design, the forces H and S may therefore be taken asequal to V. With a heavy implement mounted behind the tractor, the wholeweight W of the tractor and implement may effectively be carried on therear tires (1).

If the direction of V is considered as radial, that of S as axial, andthat of H as longitudinal, the force H is communicated to the wheel by atorque at the axle having a magnitude H x r, where r is the rollingradius of the tire (1). The number n and diameter d of the bolts holdingthe wheel to the axle flange (3) at a pitch circle of radius x have tobe sufficient to carry the torque H x r. It will therefore beappreciated that the shear strength Q of each bolt will need to begreater thannQ =r/xH.

A typical large tractor tire has r=860 mm and x=138 mm, from which itcan be seen that nQ =approximately 6.2 V, because H is approximatelyequal to V. It can therefore be concluded that conventional wheels arefixed to the axles by an arrangement of bolts which, if they are strongenough to carry the torque, are several times stronger than is requiredto carry the vertical loads. This applies to a wide range of wheels usedon most vehicles, including passenger cars.

As a result, if such a wheel when mounted to a vehicle runs over a mine,the resulting explosion applies a generally vertical load to the wheel,which becomes large enough to destroy the wheel, damage the axle andtransmit shock loads into the vehicle large enough to kill or injure theoccupants of the vehicle.

Referring now to FIG. 2, a wheel assembly of a first embodiment of thepresent invention has a wheel rim (10) welded to a first support platein the form of nave plate (11) which has a generally circular hole 12 inits center, which is a close fit on a second coupling device in the formof a shear bolt (13). A second engaging device in the form of two keys(14, 15) are provided at diametrically opposite locations across thenave plate (11), the two keys (14, 15) being a sliding fit withincorresponding key ways (16, 17) cut in the face of torque transferdevice provided as a body member in the form of a torque ring (18). Afirst coupling or engaging device in the form of two further key ways(19, 20) are cut on the opposite face of the torque ring (18), the keyways (19, 20) being located on a diameter generally at right angles tothe diameter through key ways (16, 17).

A second support plate or drive plate (21) carries keys (22, 23) forslidably engaging key ways (19, 20) on torque ring (18). The drive plate(21) also has a central boss (24) which protrudes towards nave plate(11), a distance slightly less than the thickness of torque ring (18). Athreaded hole (25) in the boss (24) receives the thread of shear bolt(13) so that the shear bolt (13) can be tightly screwed into hole (25)to hold the wheel assembly together to form a single wheel having acomposite nave plate across the face of the rim (10). This assembly canthen be bolted to a vehicle axle flange (26) by means of removablesecuring devices in the form of bolts passing through holes (27, 28) inthe drive plate (21) and the axle flange (26) respectively. A set oflarger holes (29) is also provided in nave plate (11) to allow a boxtype spanner (not shown) to be used to tighten the nuts or bolts used onthe axle flange (26).

As shown in FIG. 3, which shows a side cross-sectional view through thewheel assembly of FIG. 2 but having a tire (30) mounted on the wheel rim(10), it can be seen that in the position shown with one of the pairs ofkeys (14, 15) or (22, 23) arranged vertically, only the shear bolt (13)prevents the torque ring (18) sliding upwards relative to the driveplate (21) or the nave plate (11) sliding upwards relative to the torquering (18). When the corresponding key ways (16, 17) or (19, 20) areangled relative to the ground, there is some friction between the keysand the corresponding key ways, and it is found that when an explosionoccurs, the friction is mainly due to the inertia of the torque ring(18), which should therefore be kept as light as possible.

FIG. 3 shows typical dimensions of a large tractor wheel and tire (30),and provides a basis for demonstrating a way in which the single shearbolt (13) supports the vertical load V and also either an outwardsideways load O or an inward sideways load I when the tractor is workingnormally and shears off at an appropriate load as a result of anexplosion.

The inward load I puts compression into the drive plate (21) via thetorque disc (18) producing the outward force D on the nave plate (11)and a tensile force B in the shear bolt which can be readily calculatedas 400/460I or 0.87 I.

When the sideways force is outward, O puts the assembly of plates at thetop into compression at E and the shear bolt is in tension of1260/400×O=3.15×O. The maximum value of O is approximately V. This isthe worst case of lateral loading.

For a wheel of the dimensions shown a 20 mm diameter Grade 8.8 bolt isappropriate. This bolt shears at about 11 ton and fails in tension atabout 20 ton, so that the device can carry a vertical load of about 11ton, which corresponds to the maximum allowable weight of the wholetractor and implement which is carried equally on two wheels. The valueof outward side force it can carry has been calculated as 3.15×O=20 tonso that O=6.4 ton, a very appropriate value.

The single shear bolt can only supply the correct amount of shear forthe vertical loads and enough tensile strength for the sideways loadsfor wheels in which the torque ring is large relative to the tirediameter. When it is smaller, it is necessary to add small diameterbolts through the torque ring 18 and drive plate 21 and through holeswith much greater clearance in the nave plate 11. These bolts can bearranged to fail one after the other after the main shear bolt hasfailed.

When the shear bolt (13) fractures, the wheel leaves the tractor cleanlyand it is only necessary to unscrew the broken piece of shear bolt (13)and collect the undamaged torque ring (18) and fit a new wheel and tire(30).

Referring now to FIG. 4, in which parts common to the embodiment ofFIGS. 2 and 3 are denoted by like reference numerals but increased by100, a second embodiment of the invention, suitable for use with one ofthe front wheels of the tractor of FIG. 1, is shown. It has been foundthat an explosion under a tire has a considerable outward component aswell as the main upward force, and the rim (110) of a typical tractorwheel is quite thin, as a result of which the explosion breaks the weldbetween the rim (110) and the nave plate (111) at the bottom and goes onto drive the bottom half of the rim (110) up into the top half, formingan inverted U which is projected outwards and upwards from the tractor.In order to accommodate a gear box (131) via which the wheel of FIG. 4is mounted to an axle flange (126), the drive plate (121) is providedwith a generally cylindrical extension (132). A second coupling devicein the form of shear bolt 113 and a first coupling device in the form oftorque ring 118 are also shown.

In order to accommodate a gear box (131) via which the wheel of FIG. 4is mounted to an axle flange (126), the drive plate (121) is providedwith a generally cylindrical extension (132).

The arrangement shown in FIG. 4 has the disadvantage that the wheel isset outwards a considerable distance from gearbox (131), which may notbe strong enough to withstand the bending this imposes. A thirdembodiment of the invention shown in FIG. 5 can be used. In thisarrangement the wheel is brought inwards so that its center is in linewith the axle flange 226.

Referring now to FIG. 5, in which parts common to the embodiment ofFIGS. 2 and 3 are denoted by like reference numerals but increased by200, a third embodiment of the invention is shown. This embodiment isintended to prevent the wheel rim (210) wrapping around the drive plate,which is now an assembly consisting of parts (221), (233), (236) and(238) and holding the rim (210) to the axle flange (226), increasing thetransfer of load to the tractor and making the fitting of a new wheelmore difficult. The drive plate (221) has a generally conical shape,having as steep an angle as possible. An additional plate (234) isprovided towards the outside of the rim (210) and carries vertical andhorizontal loads.

The torque into the wheel is applied through a separate flange (235),set as far into the wheel as possible, via torque ring (218) to a flange(236) fixed to the conical extension of drive plate extension (221).When an explosion occurs under the tire and shear bolt (213) fails, therim (210) moves upwards closing gap (237) between the rim (210) andconical extension of drive plate (221). The rim (210) then slides alongthe conical surface of drive plate (221) under the combined effects ofthe conical shape and the outward component of the blast. A cylindricalextension (238) is provided to prevent the collapsing rim (210) frombecoming trapped behind the flange (236).

Referring now to FIG. 6, a fourth embodiment of the invention is shown,in which parts common to the embodiment of FIGS. 2 and 3 are denoted bylike reference numerals but increased by 300. The torque ring (18, 118,218) having two pairs of key ways is replaced by a torque plate (340)having straight line linkages arranged at right angles to each other.This arrangement transmits torque from the drive plate (321) to the naveplate (311), without unduly restricting radial movement of the naveplate (311) relative to the drive plate (321).

Links (339, 341) of generally equal length L, are pivoted to the torqueplate (340) by respective bolts through holes (342, 343). The centers ofthese bolts are placed generally equidistantly from shaft axis (344) ondiameter line (345). The other ends (346, 347) of the links (339, 341)are pivoted to the drive plate (321) at pivot points (348, 349)respectively, which also lie on a diameter of the drive plate (321) andare generally equidistant from the shaft axis (344). As a result, links(339, 341) are generally parallel to each other and at right angles todiameter line (345) when the center point X of diameter (345) coincideswith the shaft axis (344). This arrangement allows the center X oftorque plate (340) to move in a straight line over short distances alongdiameter (345) on either side of the shaft axis (344). The distance overwhich this movement approximates to a straight line depends upon thelength L of the links (339, 341), and in practice, these links should bemade as long as practicable.

A similar arrangement is provided by links (350, 351) which connect thetorque plate (340) to the nave plate (311). Diameter line (352) on whichlinks (350, 351) are pivoted to the torque plate (340) is arrangedgenerally at right angles to diameter line (345). This arrangementallows the nave plate (311) to move a short distance in a radialdirection on either side of the shaft axis (344). The combined action ofthe straight line link system described in this embodiment simulates themovements permitted by the orthogonal key ways of the embodiment of FIG.2, provided that the movements involved are small.

First coupling member in the form of shear bolt (313) is a close fit inhardened bush (353), which is fixed to a hole (354) in the nave plate(311). As in the previous embodiments, torque from the axle flange (326)is transmitted to nave plate (311) without significantly restricting theradial movement of the nave plate (311). The flange, torque and naveplate assembly are held together by two pairs of second coupling membersin the form of bolts (355), of which only one bolt is shown in FIG. 6.These bolts (355) pass through enlarged holes (356) in the nave plate(311) and therefore do not restrict the radial movements of the naveplate (311). Typically, the shank of this bolt (355), in the regionwhere it acts as the pivot (348, 349), has a diameter required to carrythe heavy torque loads to the links (339, 341). The diameter of thisbolt, in the region where it passes through the corresponding enlargedhole (356), is reduced so that is just sufficient to carry the smalltensile load holding washer (357) against the face of nave plate (311)to clamp the plate assembly together.

With this arrangement, when the shear bolt (313) fails as a result of anexplosive load, bolt (355) and the other three corresponding bolts (355)will shear off easily allowing the nave plate (311) to separate cleanlyfrom the drive plate (321) without putting an undue load on the axleflange (326).

It will be appreciated by persons skilled in the art that the aboveembodiments have been described by way of example only, and not in anylimitative sense, and that various alterations and modifications arepossible without departure from the scope of the invention as defined bythe appended claims.

1. A wheel assembly for a vehicle, the assembly comprising: a wheel rimadapted to support a tire; a first support plate mounted to said rim; asecond support plate for mounting to an axle; at least one firstcoupling device, connected between said first and second support plates,for transferring torque between said first and second support platesand, when said first coupling device is engaged with said first andsecond support plates, prevention of radial displacement of the firstsupport plate relative to the second support plate is independent ofsaid at least one first coupling device; and at least one secondcoupling device, connected between said first and second support plates,for securing said first and second support plates together, said secondcoupling device preventing radial displacement of said first supportplate relative to said second support plate; wherein when radial forcesexceed a predetermined level the or each said second coupling device isincapable of transferring said radial forces from said first supportplate to said second support plate and the or each second couplingdevice enables removal of the wheel rim and said first support platefrom said second support plate.
 2. An assembly according to claim 1,wherein at least one said first coupling device includes at least onebody member having at least one first engaging device for enabling theor each said body member to slide in a first direction relative to saidfirst support plate, and at least one second engaging device forenabling the or each said body member to slide in a second direction;not parallel to said first direction, relative to said second supportplate.
 3. An assembly according to claim 1, wherein at least one saidfirst coupling device includes at least one body member and a pluralityof first linkages pivotably connected between at least one said bodymember and said first support plate, and a plurality of second linkagespivotably connected between at least one said body member and saidsecond support plate.
 4. An assembly according to claim 1, wherein atleast one said second coupling device further includes at least onefirst coupling member connected between said first and second supportplates and adapted to fracture as a result of application thereto ofradial or axial forces exceeding said respective predetermined levels.5. An assembly according to claim 4, wherein at least one secondcoupling device includes at least one second coupling member adapted tofracture as a result of fracture of at least one said first couplingmember.
 6. An assembly according to claim 1, wherein said second supportplate is mounted to said axle via a gearbox.
 7. An assembly according toclaim 1, wherein said second support plate defines a region ofdecreasing cross section in a direction transverse to said axle.
 8. Anassembly according to claim 1, further comprising at least one removablesecuring device for securing said assembly to the axle.
 9. A vehiclecomprising: (i) a chassis; (ii) a cab for accommodating at least oneoccupant; (iii) at least one wheel assembly comprising: a wheel rimadapted to support a tire; a first support plate mounted to said rim; asecond support plate for mounting to an axle; at least one firstcoupling device, connected between said first and second support plates,for transferring torque between said first and second support platesand, when said first coupling device is engaged with said first andsecond support plates, prevention of radial displacement of the firstsupport plate relative to the second support plate is independent ofsaid at least one first coupling device; and at least one secondcoupling device, connected between said first and second support plates,for securing said first and second support plates together, said secondcoupling device preventing radial displacement of said first supportplate relative to said second support plate; wherein when radial forcesexceed a predetermined level the or each said second coupling device isincapable of transferring said radial forces from said first supportplate to said second support plate and the or each second couplingdevice enables removal of the wheel rim and said first support platefrom said second support plate; and (iv) a respective axle provided onthe chassis for supporting the or each said wheel assembly.